Sound collecting system



Aug. 6, 1940. E. w. KELLOGG SOUND COLLECTING SYSTEM Filed Dec. 31, 1937 3 mentor Patented Aug. 6, 1940 UNITED STATES 2,210,415 SOUND COLLECTING SYSTEM Edward W. Kellogg, Moorestown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 31, 1937, Serial No. 182,721 '5 Claims. (Cl. 1810.5)

This invention relates to sound collecting systems, and more particularly to a microphone system of the directive type.

Many attempts have been made to P de a microphone which is sensitive to sound comin from a particular source and in a particular direction and insensitive to sounds coming from other directions. Among the proposals heretofore made has been the almost obvious one of placing the microphone at the focal point of a large parabolic reflector with the reflector facing the direction from which it is desired to pick up the sound. The results have not usually been satisfactory, however, with such a system probably because, if the parabola is made large enough to give appreciable concentration of low frequency sounds, it becomes so sharply directive for high frequency sounds that it cannot be aimed sufiiciently accurately. For example, the directive characteristics of such a system show that, at the high frequencies, certain components will be cancelled out if the source is not exactly on the axis. One attempt to lessen this effect at the high frequencies consists in using a parabola of smaller base diameter and of considerable depth or length. However, it has been found that with such a reflector, objectionable resonances occur. When deep reflectors have been used, resort has been had to attaching Helmholtz 0 resonators near the microphone so that the excess sound will be absorbed at certain frequencies. Another fault which is almost always encountered when sound concentrators of the lens or the parabolic reflector type are used is that the high frequency components are strongly reinforced while those of lower frequency are less afiected, with the result that the sound picked up by the microphone is badly unbalanced. If the reflector is made large enough to be effective at low frequencies, the concentration of the high frequency components is still further increased.

The primary object of my present invention is to provide an improved sound collecting system of the type under consideration which will be free from the foregoing and other similar defects of prior art systems.

More specifically, it is an object of my present invention to provide an improved sound c01- lecting system as aforesaid which will be sufficiently directive for both high and low frequency sounds without having such extreme directivity at certain frequencies as to make it diificult to secure good results.

Another object of mypresent invention is to provide an improved sound collecting system which need not be aimed with extreme accuracy toward the sound source in order to receive sound therefrom in the desired direction.

Still another object of my present invention is to provide an improved sound collecting system 5 of the type described which will have a characteristic free from objectionable resonant peaks.

It is also an object of my present invention to provide an improved sound collecting system as aforesaid which is extremely simple in construction and highly eflicient in use.

In accordance with my present invention, I employ a linearly disposed wave propagating medium along which audio frequency waves are transmitted with a velocity substantially equal to the velocity of sound in free air. I make provision for a certain degree of coupling between said wave propagating medium and the surrounding air such that sound waves in the surrounding air will give rise to sound waves in the linear propagation medium. This coupling may be continuous or at a series of discrete points, such points being preferably spaced several per wave length of the highest frequency sounds to be collected. If a sound wave in the surrounding air is traveling parallel to the transmission medium, a wave is started in said medium, and this wave becomes reinforced at each point of coupling and builds up to relatively large amplitude by the time it reaches the end of the medium. If, on the other hand, the external wave is traveling at an angle to the linear medium, the small waves initiated at each coupling point are not in such phase relations to consistently reinforce each other, and only feeble waves in the medium result. In general, I provide what I term a line type of sound pick-up system, by which I mean a system in which a number of sound sensitive elements are acted upon in sequence by the wave as it progresses, provision being made for combining the outputs of these elements with proper time intervals to produce in-phase addition, provided the direction of the sound is parallel to the line along which the elements are disposed.

The waves on the line may be either acoustic or mechanical. For example, the line may consist of a hollow tubular element to one end of which is connected a suitable microphone, and the other of which is provided with suitable damping material for clamping sound waves reaching the latter end. Alternatively, the line may consist of a fine wire helix, or a long, thin wire or string subject to transverse vibrations. The sound sensitive elements may consist of a num- I ber of diaphragms or the like by which vibrations are communicatedfrom the surrounding air to the transmission line, or they may simply consist of perforations through a wall of the tubular transmission line through which sound enters an otherwise shielded chamber. The type of directive sound collecting system which is the subject of my present invention may be compared to a wave antenna, as distinguished from what I term the area type" of sound collecting system disclosed in my copending application Serial No. 182,720, filed December 31, 1937.

The novel features that I consider characteristic of my invention are-set forth with particularity .in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description of several embodiments thereof, when read in connection with the accompanying drawing, in which:

Figure 1 shows one modification of a sound collecting system employing a tubular transmission line according to my present invention,

Figure 2 is ,a similar view of a modified form of this type of sound collecting system,

Figure 3 is a modification of my invention employing a fine wire helix as the transmission line,

Figure 4 is still another modification wherein a transversely vibratable transmission line is employed,

Figure 5 is a view, partly in plan and partly in section, of still another form of my invention, and

Figure 6 is a sectional view thereof taken on the line VI-VI of Figure 5.

Referring more particularly, to the drawing, wherein similar reference numerals designate corresponding parts throughout, I have shown, in Fig. 1, a tubular member or pipe I of relatively great length and small diameter provided with a series of longitudinally spaced, small openings 3 along its length by means of which communication is established between the atmosphere and the interior of the tubular member I, the small air columns in the openings 3 serving as the coupling means between the atmosphere and the interior of the tube I. The diameter of the tube I should be small in comparison to the shortest wave lengths to be received, while the length should be of the order of a wave length or more of the lowest frequency sounds to be received. Coupled to the end of the tubular member I adjacent the openings 3 is a suitable microphone or other sound responsive device 5, while the opposite end of the tube or pipe I is preferably closed by a cover I and filled with a suitable sound absorbing or damping material 9, a sufiicient length being filled with such absorbing material to substantially prevent reflection of sound waves reaching said end.

' A sound wave such as that shown at A in Fig. 1 travelling through the atmosphere in the direction of the arrow and substantially parallel to the length of the pipe I will establish waves inside the pipe I through the openings 3, and, if the sound waves within the pipe I travel at the same velocity as the waves outside of the pipe, or in the atmosphere, those inside of the pipe I are built up or augmented'by additional energy communicated from without through each of the openings 3 as they progress toward the microphone 5. The damping material 9 is provided so that waves built up within the pipe I in the opposite direction will be absorbed. Very small openings 3 will sufiice to build up a reasonable intensity at the microphone 5. For example, in

an experimental model, I used a pipe 6 feet long by 1 inch in diameter with inch holes every inch. If the openings 3 are made too large, a high intensity of sound is provided at the microphone 5 regardless of the direction of the sound wave. The openings 3 should be so small that only the cumulated action, when the outside and inside waves travel together, will build up an .intensity within the tube approaching that of the outside wave. It is also essential that the openings be so small as not to materially affect the rate of propagation of waves within the tube, nor to permit material energy loss from the inside waves. It is not necessary that the coupling shall consist of separate, spaced holes. A continuous opening or slit 2, as shown'in Figures 5 and 6, is quite as satisfactory provided it is small or narrow enough to provide only the requisite total amount of coupling.

While a-system such as that just described gives fairly satisfactory directivity, the degree of directivity obtainable therewith is much less than would be obtained were the smallcolumns of air in the openings 3 independently immune to sound coming from the side. The system shown in Fig,

2 is designed to avoid this objection. In this system, there is mounted a small, secondary diaphragm II in each of the openings 3, the diaphragms II being mechanically coupled, as by means of the pivoted bell cranks I3, to primary diaphragms I5 arranged at right angles tothe tube or pipe I. The diaphragms I5 will be acted on in sequence by the desired sound waves A and will communicate vibration to the secondary diaphragms II through the bell cranks I3, the secondary diaphragms II, in turn, causing pressure variations within the tube I. Since the velocity of sound wave propagation within the tube I is the same as that of sound waves in the atmosphere, the disturbances produced by the secondary diaphragms II will add up and reach the diaphragm of the microphone 5 (in this case illustrated as a condenser microphone) in phase, provided the sound waves A travel parallel to the tube I in the direction of the arrow. If the sound wave A travels in the opposite direction, a wave is built up in the tube I which reaches its full amplitude at the left hand end of the tube, where it is absorbed by the damping material 9, and, therefore, does not appreciably affect the microphone 5. In order that the external sound may not act on the secondary diaphragms II directly and thus offset the benefit of the directional char.- acteristics of the primary diaphragms I5, each of the secondary diaphragms II is covered with a shield I! which protects the diaphragms II from external air pressure. For the sake of convenience, the bell cranks I3 may be pivotally mounted on the shields I'I.

The principal reason for using a tubular transmission line is the ease with which equality of velocity of propagation inside and outside may be obtained. There are, however, other devices in which this condition may be realized, and one such device is shown in Fig. 3. This device should have the same directive property as those shown in Figs. 1 and 2-and employs a solid body to conduct the waves imparted to it by a number of diaphragms. The system consists of a supporting frame 2| which supports a fine helix of wire 23 by means of threads 25 of silk or the like, which discourage transverse vibrations of the helix 23. The helix 23 should have such characteristics that the velocity of propagation of longitudinal waves along it shall be approximately equal to that of ture and thus cause excessive attenuation of high frequency waves. In order to avoid filtering, the separation of the diaphragms must not exceed about 1/11- wave lengths of the highest frequency to be transmitted. Thus, if the system is to respond to all frequencies up to 6000 cycles, at which frequency the wave length would be slightly over Y helix 23.

since the the damped line.

two inches, the diaphragms should not be more than inch apart. A wave similar to the wave A in the atmosphere in passing from left to right successively actuates the diaphragms 21 which build up a longitudinal wave in the helix 23 that actuates the microphone 5 (in this case shown as of the dynamic type) carried by the framework 2| and connected to the right hand end of the The left hand end of the helix 23 is terminated in a chamber 29 filled with a suitable damping fluid 3 I, such as oil, which absorbs waves built up along the helix from right to left, the chamber 29 being also supported by the frame 2|. Where the helix 23 enters the chamber 29, a very light and flexible diaphragm 33 is attached which retains the oil or other damping fluid 3| in the chamber 29.

In order to properly damp a transmission line, it must be terminated in a resistance substantially equal to the characteristic impedance of the line. The general formula for this characteristic impedance is R: /M S, in which R is the resistance in force units for unit velocity, M is the total mass and S the elasticity or stiffness per unit length of the line structure. The wave propagation velocity is given by the formula V=\/S/M. A method of damping a transmission line which employs a continuation of the line with distributed loss of energy, as in the system of Fig. 3, is most satisfactory for preventing reflections, mechanical impedance of the damping system (in this case, an oil immersed helix) changes very slightly with changes of oil viscosity. To get the full benefit of this type of damping, it is necessary that the distributed damping shall not be great enough to materially change the characteristic impedance of the oil immersed portion as compared with the undamped portion, and

the length must be suflicient to attenuate to nearly zero any waves which travel through the damped portion and back. To meet these conditions may require an inconveniently long damped section of line, and it may therefore be desirable to employ a combination of distributed and concentrated damping. For example, in Fig. 3 I have shown the damped line terminated in a perforated plunger of very light weight which is designed to afford as nearly as practicable a resistance to motion (under average conditions as to temperature) equal to the characteristic impedance of With such a concentrated resistance at the end, a much shorter damped line will suffice. The concentrated resistance may be used alone, but the combination will, in general, function more satisfactorily, throughout the frequency range, will be easier to adjust for minimum reflection, and will be less affected by tem perature.

The length of the active portion of the line (in other words, that part to which the diaphragms are attached) will depend on the degree of directivity desired. Useful discrimination can be obtained with a length as small as a half wave length, but preferably the line should be a wave length or more long at the lowest important frequency. So far as convenience is concerned, the length of a device of this kind would, in many cases, be compensated by the fact that the other dimensions are small compared with those of the area type of directive pick-up referred to above.

In Fig. 4, there is shown a modification of my invention wherein a series of diaphragms l5, pivotally supported ona bar orthe like 35 through the bell cranks l3, produce transverse vibrations of the transmitting line, or wave propagating element. The transmission line, in this case, might consist of a long, thin bar 31 of proper stiffness and mass to have a velocity of propagation equal to that of sound waves in the air, but such a line is not altogether satisfactory because the velocity is not the same for all frequencies. A preferable form of line consists of a loaded string or very flexible wire subjected to sufiicient tension to give it the desired wave velocity. The tension can be applied to the wire or the like 31 by means of an unloaded string or wire 39, one end of which is connected to the right hand end of the member 31, and the other end of which is connected to an adjustable device, such as a wing nut and screw, 4| carried by one end of the frame 2|. The right hand end of the element 31 is also connected to a suitable microphone 5, and the left hand end thereof is terminated in a damping medium similar to that described in connection with Fig; 3, the diaphragm for confining the damping fluid in this case being designed to exert minimum restraint to transverse waves.

The primary diaphragms [5 are coupled to the transmission element 31 through the bell cranks l3 and impart transverse vibrations thereto in response to sound waves passing from left to right, as indicated by the dotted line 31, which add up to actuate the microphone 5. On the other hand, a wave passing from right to left will produce transverse vibrations in the member 31' which reach their maximum amplitude at the left hand end where they are absorbed by the damping medium 3!.

In order that the system shall not act as a mechanical filter, the diaphragms l5 must not load the string or other similar member 31 unless their spacing is a small fraction of the wave length of the highest frequency to be transmitted by the system. Whatever the spacing of the coupling elements, it is essential that they shall not load the transmission line in such a way as to cause the velocity of propagation to differ materially from the velocity of sound in air, nor to cause the velocity to be materially different at difl'erent frequencies in the audio range. As one method of preventing objectionable loading of the propagation line, I have shown, in Fig. 4, very light springs 63 interposed between the bellcranks l3 and the transmission element 31. In another form of construction, the springs may be omitted and the bell cranks l3 connected to the transmission element 31 similarly to the connections of the horizontal arms of the bell cranks l3 to the secondary diaphragms H in the modification of Figure 2. The effect of so connecting the bell cranks l3 to the bar or wire 31 would be to add a concentrated inertia load or mass at each of the points of connection between it and the several bell cranks, the added mass due to the bell cranks l3 and the diaphragms l5 being included gin the effective mass per unit from undesired directions.

length of the transmission system. In order to produce the desired velocity of propagation, the added mass would have to be compensated by increased stiffness or tension in the wire 31. It is well known that satisfactory transmission of transverse waves can be obtained in the case of a member having suitable stifiness or tension a part of the mass of which consists in concentrated masses or loads, provided the concentrated loads are close enough together to substantially duplicate the efiect of uniformly distributed mass, as in the case of the modification of Figure 4.

In all of the forms of the invention shown, the general requirements as to loading the line and spacing of the coupling elements between the atmosphere and the line are essentially the same. Apart from the filtering eilects, too great spacing of the coupling elements (diaphragms or orifices) will have the tendency to cause the system to be sensitive, at certain frequencies, to waves coming For example, if the spacing is a half wave length (at a given frequency) or a whole number of half wave lengths, waves will build up at the microphone end of the line as effectively for external Waves traveling from right to left as for waves traveling from left to right.

Although I have shown and described several embodiments of my present invention, it will be apparent to those skilled in the art that many other modifications are possible, as are also changes in those herein described. For example, in the system of Fig. 2, I have shown the diaphragms IS on both sides of the tube I and in staggered relation. This arrangement need not necessarily be followed. Many other changes will, no doubt, readily suggest themselves to those skilled in the art. I, therefore, desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A sound collecting system comprising a relatively long tubular member having a plurality of spaced, small openings along its length whereby to provide communication between the interior of said tubular member and the atmosphere, and a microphone connected to one end of said tubular member and arranged to be actuated by sound waves set up in said tubular member, said openings affording an inlet into said tubular member for sound waves in the atmosphere surrounding said member whereby a wave passing along said tubular member sets up vibrations therein through said openings.

2. A sound collecting system according to claim 1 characterized in that tubular member is of 'the order of a wave length long for the lowest frequency sounds intended to be picked up by said system, and characterized further in that said openings are so spaced apart that the waves set up in said tubular member through each of them will be in phase.

3. A sound collecting system comprising a relatively long tubular member having a plurality of spaced, small openings therein along its length, a diaphragm closing each of said openings, a second diaphragm associated with each of said first named diaphragms, said second diaphragms being arranged outside of said tubular members and disposed in a manner to be successively acted upon by a sound wave travelling through the at= mosphere, means coupling said second named diaphragms to their respectively associated first named diaphragms whereby actuation of said second named diaphragms will effect actuation of said first named diaphragms to set up pressure variations within said tubular member, and a microphone connected to one end of said tubular member and arranged to be actuated by the waves set up in said tubular member in response to actuation of said first named diaphragms.

4. A sound collecting system comprising a relatively long tubular member having a plurality of spaced, small openings therein along its length, a diaphragm closing each of said openings, a second diaphragm associated with each of said first named diaphragms, said second.- diaphragms being arranged outside of said tubular members and disposed in a manner to be successively acted upon by a sound wave travelling through the atmosphere, means coupling said: second named diaphragms to their respectively associated first named diaphragms whereby actuation of said second named diaphragms will eifect actuation of said first named diaphragms to set up pressure variations within said tubular member, a microphone connected to one end of said tubular memher and arranged to be actuated by the waves set up in said tubular member in response to actuation of said first named diaphragms, and sound wave absorbing means at the other end of said tubular member for absorbing waves reaching said last named end.

5. A sound collecting system according to claim 4 characterized by the addition of means for shielding each of said first named diaphragms against direct actuation thereof by sound waves in the atmosphere.

EDWARD W. KELLOGG. 

